surface should be covered with a 1:1 mixture of cooking oil and patrolium jelly, D
In Finland that is old recipe. Too complicated.
Most use screen bottom, and under it it is a plate.
I have in every hive mites (17 years) and I need not to know how much I have mites. I give every year oxalic acid on them. From bottom I can then see, how much mites have died, hudreds or thousands.
When I cut away Also from drone pupas you can evaluate how much they are in the hive.
DON'T PANIC WITH MITES!